Identifying uncertainty in role of aviation NOx emissions

Scientists study the uncertainty in the impact of aviation emissions of …

Climatologists continue to develop and refine predictive models for climate change, attempting to limit uncertainty. A lot of attention is focused on carbon dioxide emissions, but a major source of uncertainty is the role of nitrogen oxides (NOx, the sum of NO and NO2), which promote greenhouse gas formation. In addition to other sources, these are emitted by aircraft engines in the upper atmosphere. A paper that recently appeared in Proceedings of the National Academies of Science studied the complex influence of NOx and describes the processes that need to be studied further to reduce our uncertainty about the impact of NOx emissions.

NOx emissions from combustion contribute on the ground to smog and acid rain, and in the upper atmosphere they act as an indirect greenhouse gas by causing a short-term increase of ozone (O3), an important greenhouse gas. Atmospheric ozone is eliminated through reactions with the hydroxyl (OH) and hydroperoxyl (HO2) radicals, and NO interferes with these reactions by competing for HO2, leading to an increase in ozone. This increase in ozone causes a greenhouse forcing, an increase in the amount of energy retained by the atmosphere.

In the long term, emissions of NOx lead to reduced levels of methane (CH4), another potent greenhouse gas. Due to the complex relationships between chemical species in the atmosphere, the loss of methane also leads to a longer-term decrease in ozone. These decreases lead to negative greenhouse forcing, causing atmospheric cooling.

Currently, our uncertainty lies in the relative amount of negative and positive forcing, which are both large and almost cancel out. Most past studies showed that NOx emissions result in a net positive forcing (and therefore emissions tend to cause global warming), but other studies showed a net negative forcing.

<!--The authors of the paper took multiple recently-published models and calculated the resulting radiative forcing due to 1.0 Tg/yr of NOx emissions (the actual estimated emissions ranges from 0.4-1.3 Tg/yr). Then, taking the resulting values as samples in a normal distribution, they found the following radiative forcing effects: +27.3 ± 9.7 mW/m2 from short-term O3 increase, -16.1 ± 5.6 mW/m2 from long-term CH4 reduction, and -6.6 ± 3.3 mW/m2 from long-term O3 reduction. Taken together, the net forcing is +4.5 ± 4.5 mW/m2.
--> The authors of the paper took multiple recently published models and calculated the resulting radiative forcing due to 1.0 Tg/yr of NOx emissions (estimates of emissions range from 0.4-1.3 Tg/yr). Then, taking the resulting values as samples in a normal distribution, they found the net radiative forcing is +4.5 ± 4.5 mW/m2. That would imply a likely warming effect, although the impact could be quite small.

Using this approach, however, the sources of uncertainty cannot be determined. To get at those, the authors used an approach called factor decomposition to break down each of the sources of forcing into key components, such as the responses of O3 and CH4 due to NOx emissions, the atmospheric concentrations of the species, and the radiative forcing efficiency of each species. Another benefit of this approach is that it depends more on fundamental chemistry than modeling. Assuming each of the terms are not correlated, the resulting net radiative forcing calculated in this manner is +0.6 ± 8.3 mW/m2. This mostly agrees with the results from the set of models, although the uncertainty is larger.

The authors found that factors specifically related to aviation emissions (the response of chemical concentrations to aviation NOx) control the uncertainty in the forcing due to the short-term O3 and long-term CH4 changes. But long-term O3 impacts are influenced by how ozone and methane interact in the atmosphere (which isn't aviation specific).

Another interesting result in the paper is that the positive greenhouse forcing due to the short-term increase in O3 and the negative forcing due to long-term CH4 reduction appear to be strongly negatively correlated (when one goes up, the other goes down). If this is taken into account in the uncertainty calculations, this correlation drops the uncertainties to 20 percent of the original values.

The authors suggest that background processes that control naturally produced NOx are the primary factors affecting both the uncertainty and the strong negative correlation between positive and negative forcings. For instance, lightning is a large source of NOx (4 Tg/yr based on one study), and the impact of aviation emissions will be influenced by this value. A better understanding of this and other natural processes will reduce the uncertainty when it comes to aviation NOx, as will further studies focusing on the factors identified by the authors of this paper.

Kyle Niemeyer
Kyle is a science writer for Ars Technica. He is a postdoctoral scholar at Oregon State University and has a Ph.D. in mechanical engineering from Case Western Reserve University. Kyle's research focuses on combustion modeling. Emailkyleniemeyer.ars@gmail.com//Twitter@kyle_niemeyer

14 Reader Comments

I'd be interested to know the definitions of "short-term" and "long-term" in relation to this study. Are we talking a likely net positive forcing on a scale of days, weeks, years? Climatology typically deals with at the very least a multi-decade time scale; I'm sure the details are in the full paper, but I can't get to that myself.

It's also good to know we haven't been dumping a whole load of gasses not normally present in the atmosphere. While the quantity is considerable, at least it's not the majority source of these gasses (re the quote about lightning producing at least 4 times more)

We simply need to find a way of reducing the number of lightning strikes and we can fly around as much as we want!

I'd be interested to know the definitions of "short-term" and "long-term" in relation to this study. Are we talking a likely net positive forcing on a scale of days, weeks, years? Climatology typically deals with at the very least a multi-decade time scale; I'm sure the details are in the full paper, but I can't get to that myself.

They don't really give many details on this, but the short-term ozone perturbations are on the order of months while the long-term methane and ozone perturbations are on the order of years.

Quote:

In any case, the chemistry and complex interactions are fascinating.

Thanks! I thought about giving more on the long-term ozone-methane interaction, but it's difficult without getting into reactions.

It's also good to know we haven't been dumping a whole load of gasses not normally present in the atmosphere. While the quantity is considerable, at least it's not the majority source of these gasses (re the quote about lightning producing at least 4 times more)

We simply need to find a way of reducing the number of lightning strikes and we can fly around as much as we want!

use Tesla's solution- it is being tested out in a desert somewhere still- nevermind.

concentrations small enough not to rise or fall?

science- look at the contrails... rest not- good try though

Modeling must use inputs that proves itself write- in order to survivefunding scrutiny.

Another interesting result in the paper is that the positive greenhouse forcing due to the short-term increase in O3 and the negative forcing due to long-term CH4 reduction appear to be strongly negatively correlated (when one goes up, the other goes down). If this is taken into account in the uncertainty calculations, this correlation drops the uncertainties to 20 percent of the original values.

I'm more interested in understanding this from a statistical standpoint than a chemical. I understand (in principle) how these forcings could be related based on the chemistry, but lack the statistical background to understand why this results in reduced uncertainty. The wording of the first sentence seems to me to imply that it would vary more wildly, as the presence of the positive forcing reduces the strength of the negative. Either I'm misinterpreting that line, or I don't understand how the math is working.

It's also good to know we haven't been dumping a whole load of gasses not normally present in the atmosphere. While the quantity is considerable, at least it's not the majority source of these gasses (re the quote about lightning producing at least 4 times more)

Wouldn't lighting-created NOx would be an effective constant across climate modelling, and as such the human-caused change to the amount created annually - possibly by as much as 25% if lightning was the only natural source and created 4 times the amount as airliners - be the major factor in NOx's role in changing the climate (if it had such as effect, this article suggesting it is minimal)?

Your points are valid - at least NOx gases are naturally occurring in large quantities, and thus life is used to them - unlike some novel toxic gas, and at least we're not pumping out double the amount which would normally be created in a year. I'm just being pedantic, largely due to a similar argument RE: CO2 and volcanoes being used in climate change denial posts.

It's also good to know we haven't been dumping a whole load of gasses not normally present in the atmosphere. While the quantity is considerable, at least it's not the majority source of these gasses (re the quote about lightning producing at least 4 times more)

Wouldn't lighting-created NOx would be an effective constant across climate modelling, and as such the human-caused change to the amount created annually - possibly by as much as 25% if lightning was the only natural source and created 4 times the amount as airliners - be the major factor in NOx's role in changing the climate (if it had such as effect, this article suggesting it is minimal)?

Your points are valid - at least NOx gases are naturally occurring in large quantities, and thus life is used to them - unlike some novel toxic gas, and at least we're not pumping out double the amount which would normally be created in a year. I'm just being pedantic, largely due to a similar argument RE: CO2 and volcanoes being used in climate change denial posts.

Its a perfectly valid point; sure, our emissions of NOx and CO2 are both small compared to the natural processes, but the natural processes also know how to "handle themselves" as it were. For example:

The human additions are potentially disruptive because the climate system doesn't account for them, quite obviously in the case of CO2, not so clearly in the case of NOx.

I understand why the vegetation and land show CO2 both absorbed and emitted. But I'm not at all clear about the oceans. Are the oceans at the higher latitudes absorbing CO2 while the tropical waters are out-gassing CO2? How does this work.

Also, what's the breakdown of 29(GT?) "fossil fuel burning + land use", that is, how much for each?

IIRC, Archeabacteria create massive quantities of methane and co2 emissions, blue-green algae take in co2 and create molecular oxygen. The water itself, in a purely chemical reaction, also absorbs a certain amount of CO2 from the atmosphere, increasing the acidity of the water.

Not sure of the exact chemical equations off the top of my head, though.

edit: http://crecherche.ulb.ac.be/facs/scienc ... tt2003.pdfalso, removed bit about red algae and blooms. I forget the overall balance of co2 to o2 production in the light/dark reactions during a bloom. They do create dead spaces with massively high concentrations of co2, but I can't find a good reference for overall output volume.

IIRC, Archeabacteria create massive quantities of methane and co2 emissions, blue-green algae take in co2 and create molecular oxygen. The water itself, in a purely chemical reaction, also absorbs a certain amount of CO2 from the atmosphere, increasing the acidity of the water.

Not sure of the exact chemical equations off the top of my head, though.